This invention relates to two-stage electrostatic precipitators for use in filtering air and, more specifically, to low-conductivity collector plates suitable for mounting in a manner which will increase the efficiency of such precipitators.
There are two methods to increase the performance of two-stage electrostatic precipitators. First, all of the usual bypass air leaks associated with the structural fabrication of the precipitator can be blocked off by using means which will eventually break down and lead to the electrical shorting of the high-voltage elements. Secondly, the structure of the precipitator can be improved to minimize irregularities in the air flow and electric fields which irregularities always reduce the overall air-filtration efficiency of the precipitator.
A two-stage electrostatic precipitator typically performs its function by adding air ions (usually positive) to aerosol particles in the first or ionizing stage, thereby producing a high unipolar electric charge on each aerosol particle. Then, in the second stage, the charged aerosol is passed through a closely spaced array of metal plates oriented parallel to the air flow, alternate plates being grounded while the remainder are connected to the high-voltage power source, so as to attract the charged aerosol to the metal plates.
Although the two-stage electrostatic precipitator enjoys considerable use in home, commercial, and industrial installations, there is a major problem which restricts their wider application. This problem is the occurrence of spark discharges between the charged and grounded plates when the spacing between the plates is effectively reduced. There are several ways in which this reduction or narrowing of the airspaces between the plates can occur. The narrowing may be caused by the introduction of a fiber or needle-like single particle of dust or lint, the accumulation of smaller particles in the interplate field into a chain (this is a well-known occurrence with metal or carbon particles, but also occurs with thin materials), and the general building up of bulk deposited aerosol until the interplate spacing becomes small enough that the point-to-point electrical breakdown distance for air is reached.
Such spark discharges are in certain cases intolerable, ruling out the use of electrostatic precipitators in applications for which they would otherwise be well-suited. Such an undesirable case occurs where the dust of aerosol deposit is flammable, as with pyrophoric metals, greasy materials, or even some house dust. While in other situations the interplate sparking is not intolerable, it is undesirable nonetheless because it causes the production of irritant, toxic ozone, and nitrogen oxides, the disposal of dust deposits due to the explosive effects of the spark, and a firecracker-like noise of the spark.
Electrostatic precipitator plate electrodes which would perform all of the functions of the usual metallic electrodes, yet be non-sparking, would substantially enlarge the range of application of the two-stage precipitator, and eliminate the objectionable interplate discharge in present precipitators. In addition, such plates constructed of a suitable material and design and properly mounted eliminate the bypass air leaks found in prior-art electrostatic precipitators.
Therefore, it is an object of the present invention to improve the efficiency of the second stage of an electrostatic precipitator by the elimination of bypass air leaks.
Another object of the present invention is to eliminate the possibility of sparking in the second stage of an electrostatic precipitator.
Still another object of the present invention is to provide a simply constructed and inexpensive second stage for an electrostatic precipitator that can be used in environments where sparking between the elctrodes is intolerable or undesirable.